1. Field of the Invention
The present invention relates to papermaking and, in particular, continuous circulation and supply systems for paper web coaters and size presses.
2. Description of the Prior Art
To enhance the printing qualities of paper and paperboard, one or both surfaces may be coated with a fluidized blend of finely particulated minerals and adhesive, dried and calendered. The predominate mineral constituent is coating clay blended with small quantities of calcium carbonate and/or titanium dioxide. This coating blend is traditionally applied to a traveling paper web by one of numerous coating application systems. Several of these coating application systems include a standing flow pond or channel of coating fluid confined on one side by the traveling web surface whereby the traveling web is drawn through the standing pond of coating material. Representative of such systems are blade coaters, roll coaters and air doctors. Supporting such an application system is a pumped circulation flow rate that may be ten times greater than the web surface application rate.
Although these flooded channel or standing pond application systems effectively transfer a substantially uniform distribution of coating fluid to the web surface, simultaneously, the web surface is being flushed of loose lint, fiber bundles and accumulated contaminants. Other foreign matter such as grit, ash, chips, string and bits of kraft or jute bags can be introduced to the coating fluid circulation system with the raw materials. Undispersed or oversize pigments and flocculated or undissolved adhesives can come from the coating preparation and dried coating may flake from the coating apparatus and fall back into the standing pond.
Foreign and contamination particles larger than 100 microns will cause coating streaks and blemishes with most application systems and particles larger than 45 microns may create blemishes with a blade coating apparatus. To control and remove such disruptive contamination, coating fluid circulation systems are provided with mixers, dispersion equipment and strainers or screens. Although helpful and even essential, the prior art screening devices are less than satisfactory. Selection and sizing are influenced by numerous factors such as system flow rate, fluid viscosity and acceptable particle size.
Totally enclosed pressure screens are preferred to avoid the air entrainment incident of atmospherically open screens. However, the finest screen mesh practical with a pressure screen is about 150 Tyler mesh (105 micron). Blade coating apparatus requires a 44 micron (325 Tyler mesh) acceptable particle size. Finer mesh pressure screens occasion unreasonably short plugging and cleaning cycles.
Atmospherically open, self-cleaning screens may be sized to accommodate a large flow rate and fine particle size. However, this type of screen also entrains air to foam the circulation system.
It is, therefore, an object of the present invention to provide a paper web coating fluid circulation system of high flow rate that is screened of most particles greater than 44 microns.
Another object of the present invention is to provide a 325 Tyler mesh screen for paper web coating systems.
Another object of the present invention is to provide a deaeration system for 325 Tyler mesh screened coating fluid.